Effects of Ni precursors on the formation of Mg–Fe–Ni intermetallic hydrides, kinetics, and reversibility

This work focuses on the effects of Ni precursors (metallic Ni or Mg2NiH4) on the formation of Mg–Fe–Ni intermetallic hydrides as well as their de/rehydrogenation kinetics and reversibility. After ball milling and sintering, the formation of Mg2FeH6 and Mg2NiH4 are found in both samples, while MgH2 is observed only in the sample with metallic Ni. Both samples show comparable hydrogen capacities of 3.2–3.3 wt% H2 during the 1st dehydrogenation, but the sample with metallic Ni decomposes at a lower temperature (ΔT = 12 °C) and shows faster kinetics. Although phase compositions after dehydrogenation of both samples are comparable, their rehydrogenation mechanisms are different. This affects the kinetic properties upon cycling and reversibility. Reversible capacities of the samples with metallic Ni and Mg2NiH4 during the 2nd dehydrogenation are 3.2 and 2.8 wt% H2, respectively, while those during the 3rd–7th cycles reduce to ∼2.8 and 2.6 wt% H2, respectively. Chemical and microstructural characterizations are carried out to explain de/rehydrogenation pathways.


Introduction
The intermetallic hydride of Mg 2 FeH 6 has been considered for hydrogen storage applications due to the highest volumetric hydrogen density (150 kg m −3 ) and relatively high gravimetric hydrogen density (5.5 wt% H 2 ). 1,2 Also, its high reaction enthalpy (∼90 kJ per mol H 2 ) as well as high volumetric and gravimetric energy densities (0.49 kW h L −1 and 0.55 kW h kg −1 , respectively) are suitable for the thermochemical energy storage medium. 1,3-6 However, Mg 2 Fe intermetallic alloy is thermodynamically unfavorable and the signicant difference in density and melting points of Mg and Fe hinders the formation of homogeneous alloys via metallurgical methods. 7,8 Several procedures, such as thermal processes, mechanical milling, cold rolling, and high-pressure compressions have been applied to Mg + Fe or MgH 2 + Fe mixtures for Mg 2 FeH 6 syntheses. [9][10][11][12][13][14][15] Hydrogenation of Mg to MgH 2 catalyzed by Fe was rst found at ∼200°C and the obtained MgH 2 further reacted with Fe to form Mg 2 FeH 6 only at high temperature (∼350°C) due to kinetic restriction from solid-solution diffusion processes. 16,17 Moreover, reversibility of Mg 2 FeH 6 via the reaction between MgH 2 and Fe required high operating temperatures (T = 375-445°C) to achieve reasonable hydrogen capacity. 9,15 Quaternary intermetallic hydrides via partial substitution of transition metals (TMs) for Fe in Mg 2 FeH 6 to form Mg 2 Fe (1−x) TM x H 6 (TM = Cr, Ni, Mn, Co, and Y) have been proposed to enhance kinetics and reversibility. The samples were prepared by (i) milling MgH 2 with the plain steel containing TM impurities (e.g., 316L stainless steel and g-Fe(Ni) nanoparticles) 8,18,19 and (ii) compositing TMs in metallic form or compounds with Mg + Fe, MgH 2 + Fe, or Mg 2 FeH 6 . 20-25 These processes increased Mg 2 FeH 6 yield with the improved kinetic properties and reversibility. Immediate reaction between MgH 2 and 316L SS via either reactive ball milling under hydrogen pressure or ball milling under Ar atmosphere and annealing under hydrogen pressure resulted in partial substitution of Fe with Cr and Ni to form Mg 2 (Fe, Cr, Ni)H x . 8,18 Such a faster reactivity with respect to pure iron was induced by martensitic transformation during ball milling and the presence of Ni in the system. Moreover, Mg 2 Fe(Ni)H 6 with tangled nanowire morphology prepared using coarse-grained Mg powder and g-Fe(Ni) nanoparticles showed lower desorption temperature by 20°C as compared with Mg 2 FeH 6 . 19 Catalytic effects on hydrogenation of Ni and Fe as well as comparable fcc lattice of g-Fe(Ni) and Mg 2 FeH 6 , shortening Fe diffusion distance favored the formation of Mg 2 Fe(Ni)H 6 . Besides, NiFe-based catalysts favored hydrogen adsorption kinetics, resulting in the enhanced hydrogen evolution capability. 26,27 Transition metal complex deuterides of Mg 2 Fe x Co (1−x) D y (x = 0-1 and y = 5-6) prepared by reactive ball milling revealed comparable deuterium desorption temperatures at all compositions, but reversible reaction (T = 400°C under 30 bar H 2 ) with the enhanced kinetics was detected from Mg 2 Fe 0.5 Co 0.5 H 5.5 . 21 Theoretical studies reported destabilization of Mg 2 FeH 6 , i.e., reduction of formation energy and desorption temperature via substitution of Fe with Ni, Co, and Mn. 20 The most signicant reduction of desorption enthalpy was expected from Mg 2 Fe 0.75 Ni 0.25 H 6 (27.7 kJ per mol H).
Among Mg-Fe-TM intermetallic hydrides, Mg-Fe-Ni-H system shows remarkable hydrogen sorption kinetics, meanwhile all metallic compositions (Mg, Fe, and Ni) are inexpensive. From our previous work, Mg 2 Fe 0.75 Ni 0.25 H 6 formed during dehydrogenation of 20 wt% Ni-doped Mg 2 FeH 6 showed excellent reversible hydrogen capacities with respect to as-prepared Mg 2 FeH 6 , for example, hydrogen reproduction during the 2 nd cycle increased from 78 to 85%. 23 Besides, Ni-substituted contents in Mg 2 FeH 6 was optimized by varying Mg 2 FeH 6 : Mg 2 NiH 4 mole ratios to obtain Mg 2 Fe (1−x) Ni x H 6 with the best kinetics. 25 It was found that dehydrogenation kinetics and reversibility were enhanced with Ni-substituted contents, and the most stable composition upon cycling was x ∼ 0.5 (Mg 2 Fe 0.5 Ni 0 . 5 H 6 ). From these reports, it was found that different starting materials could alter Ni substitution degree in Mg 2 FeH 6 , i.e., 25 and 26-47% for the samples prepared from metallic Ni + MgH 2 and Mg 2 FeH 6 + Mg 2 NiH 4 , respectively. In this work, we would like to extend our study on the effects of Ni precursors on the formation and reversibility of Mg 2 Fe (1−x) Ni x H 6 . Two sample sets with the same stoichiometry of x = 0.25 using MgH 2 + Fe + Ni and MgH 2 + Fe + Mg 2 NiH 4 mixtures as starting materials are ball milled and sintered under hydrogen pressure. De/rehydrogenation kinetics, reversibility, and hydrogen exchange pathways are investigated. Microstructural analyses are carried out to explain the effects of distribution and contacts among the reactive phases in nanometer range on hydrogen sorption mechanism.

Sample preparation
Mg powder ($99.0%, Sigma-Aldrich) was hydrogenated at 350°C under 38-40 bar H 2 for 12 h and milled for 1 h 30 min using a Retsch™ PM 100 Model Planetary Ball Mills. The rotational speed and the ball-to-powder weight ratio (BPR) were 500 rpm and 10 : 1, respectively. Hydrogenation and ball milling under similar conditions were repeatedly carried out until hydrogenation was complete to obtain as-prepared MgH 2 . Ni powder (99%, Alfa Aesar) was milled with as-prepared MgH 2 under 1 : 2 mole ratio using milling time, BPR, and rotational speed of 5 h, 10 : 1, and 500 rpm, respectively. Hydrogenation of 2MgH 2 -Ni mixture was done at 350°C under 40 bar H 2 for 12 h to obtain as-prepared Mg 2 NiH 4 . As-prepared MgH 2 was milled with the powder samples of Ni and Fe (99.9%, Sigma-Aldrich) with the mole ratio of 8 : 3 : 1 (MgH 2 : Fe : Ni) for 7 h 30 min using BPR and rotational speed of 15 : 1 and 500 rpm, respectively. The obtained mixture was sintered at 400°C under 38-40 bar H 2 for 48 h to obtain MgH 2 -Fe-Ni composite, denoted as S1. Fe powder was milled with as-prepared samples of MgH 2 and Mg 2 NiH 4 with the mole ratio of 6 : 3 : 1 (MgH 2 : Fe : Mg 2 NiH 4 ) and the mixture was sintered under similar condition with S1 to produce MgH 2 -Fe-Mg 2 NiH 4 composite, denoted as S2. The powder samples of S1 and S2 were heated to 500°C and rehydrogenated at 350°C under 40 bar H 2 for 12 h to obtain S1 ′ and S2 ′ , respectively.

Characterizations
Phase compositions of as-prepared and de/rehydrogenated samples were characterized by powder X-ray diffraction (PXRD) at ambient temperature using a Bruker D8 ADVANCE with Cu K a radiation (l = 1.5406 Å), a current of 40 mA, and a voltage of 40 kV. The powder sample was packed in an airtight sample holder covered with a poly(methyl methacrylate) dome in a nitrogen-lled glove box. The diffractogram was collected in the 2q range, scanning step, and acquisition time of 10-80°, 0.02°s −1 , and 400 s per step, respectively. Dehydrogenation proles were investigated by differential scanning calorimetry (DSC) and thermogravimetry (TG) using a Netzsch STA449F3 Jupiter. The powder sample of 20-30 mg was heated from room temperature to 500°C (5°C min −1 ) under N 2 ow (50 mL min −1 ). The relative signal of H 2 released from the sample was characterized by mass spectroscopy (MS) using a Netzsch QMS 403C.
X-ray photo-electron spectroscopy (XPS) experiments were carried out at the SUTNANOTEC-SLRI Joint Research Facility, Synchrotron Light Research Institute (Public Organization), Thailand. A PHI5000 Versa Probe II (ULVAC-PHI Inc., Japan) with Al Ka (1.486 keV) radiation as an excitation source was used for characterizations. The powder samples were deposited on the sample holder using carbon glue tape in the glove box. Prior to the measurements, the samples were placed in the high vacuum chamber (1 × 10 −8 mbar) for 2 h. The high-resolution scan of each element was collected using a pass energy of 46.95 eV and a step size of 0.05 eV. Dual-beam charge neutralization (low energy electron and ion beam) method was used to minimize sample charging. The binding energy was calibrated with respect to the C 1s peak (284.8 eV). The data was processed and analyzed by using a MultiPak soware version 9.6.0 (ULVAC-PHI, Japan). Peak tting was performed aer Shirley background subtraction. Symmetrical Gaussian-Lorentzian function was used to approximate the line shapes of the tting components.
De/rehydrogenation kinetics and reversibility were studied using a test station automatically controlled by the program developed in a Labview® environment. 28,29 Two K-type thermocouples (TCs, −250-1300°C, SL heater) were used to control and measure the system and sample temperatures during the experiments. Hydrogen release and supply during de/ rehydrogenation were controlled by the direct-acting plunger solenoid valves (Type 0255, Bürkert) and the system pressure was detected by a pressure transducer with an operating range of 0-3000 psig (an OMEGA Engineering PX309-3KGI). Hydrogen content desorbed was measured using a mass ow controller (MFC, 0-0.1 standard L min −1 (SLM), a Bronkhorst EL-FLOW selected F-201CV). The signals of temperature, pressure, and mass ow rate were transferred to the computer using the module data loggers (a NI USB-6009, National Instruments and an AI210, Wisco). Hydrogenation was done under isothermal condition at the setting temperature (T set ) of 315°C under 10-16 bar H 2 , while dehydrogenation was carried out at T set = 315°C by releasing hydrogen through MFC with the ow rate of 0.09 SLM. The volume of hydrogen desorbed was obtained from integrating the peak area of hydrogen ow rate (SLM) versus time (min) plots. The hydrogen storage capacity was calculated by the following equations. (2) where V STP (L) and V s (SLM) are the volumes of hydrogen gas at the standard temperature and pressure condition (STP, T STP = 273.15 K and P STP = 1.0133 bar) and at the standard condition of MFC (T s = 296.15 K and P s = 1.0156 bar), respectively. n H 2 (mol) is hydrogen moles and standard molar volume is 22.4 L mol −1 .
Morphology and microstructure were characterized by transmission electron microscopy (TEM) technique using a Thermo Scientic TALOS F200X coupled with an energy dispersive X-ray spectroscopy (EDS) micro-analysis. An accelerating voltage of 200 kV was used. Sample preparation was done by ultrasonic dispersion of the powder sample in ethyl alcohol (99% AR grade, RCI Labscan) for 10-15 min and dropping onto a carbon grid.

Results and discussion
Chemical compositions of as-prepared S1 and S2 are characterized by PXRD technique. From Fig. 1A, PXRD spectra of S1 and S2 show the diffractions of Mg 2 FeH 6 , Mg 2 NiH 4 , Fe, and MgO as well as MgH 2 and Fe-Ni alloy 30 from S1 and S2, respectively. Upon milling and sintering, the formations of Mg 2 FeH 6 and Mg 2 NiH 4 conrm hydrogenation of MgH 2 + Fe (eqn (4)) and Mg 2 Ni (eqn (5)), while that of Fe-Ni alloy is solid solution of Fe and Ni. 30 MgO is obtained from oxidation of Mgcontaining phases with oxygen and/or humidity. 2MgH 2(s) + Fe (s) + H 2(g) / Mg 2 FeH 6(s) (4) Dehydrogenation of S1 and S2 is investigated by simultaneous DSC-TG-MS experiments. From Fig. 1B, as-prepared S1 and S2 show single-step decomposition at comparable onset dehydrogenation temperatures of ∼250°C. The main desorption temperatures of S1 and S2 are 304 and 316°C, respectively. Hydrogen storage capacities of both samples are comparable in the range of 3.2-3.4 wt% H 2 (Fig. 1B). Decient hydrogen capacities with respect to pristine Mg 2 FeH 6 (5.40 wt% H 2 ) 23 and Mg 2 NiH 4 (3.4-3.6 wt% H 2 ) 31 are described by the formation of unreacted Fe and Fe-Ni alloy in as-prepared samples (Fig. 1A).
According to greater hydrogen capacities and lower dehydrogenation temperatures, further studies focus on dehydrogenation performance, reversibility, and reaction pathways of S1 and S2. Hydrogen absorption and desorption are carried out at isothermal condition (T set = 315°C) under the system pressure (P sys ) of 0-16 bar H 2 . Prior to the measurements, asprepared samples of S1 and S2 are heated from room temperature to 315°C under 15 bar H 2 to prevent dehydrogenation. Once reaching isothermal condition, dehydrogenation begins with releasing hydrogen through MFC using the constant mass ow rate of 0.09 SLM (Fig. 3). During 0-10 min, the 1 st endothermic dehydrogenation of S1 and S2 starts at the system pressure (P sys ) of ∼2 bar H 2 , conrmed by the reduction of sample temperature (T sample ) (Fig.  3). Complete Fig. 2 PXRD spectra (A) and simultaneous DSC-TG-MS results (B) after dehydrogenation at 500°C of S1 and S2 as well as S1 ′ and S2 ′ . dehydrogenation of both samples is obtained within 19-21 min, shown as the elevated T sample to the initial temperature. From  Fig. 3A, S1 reveals rapid temperature reduction to equilibrium temperature (T eq ) of 316°C under P sys = 1.13 bar H 2 with twostep decomposition, possibly belonging to MgH 2 , Mg 2 FeH 6 and Mg 2 NiH 4 . For S2, slow temperature reduction to T eq = 314°C under P sys = 0.4 bar H 2 is found with the single-step dehydrogenation of the mixed Mg 2 NiH 4 + Mg 2 FeH 6 ( Fig. 3B). At T eq = 314-316°C, the equilibrium pressures (P eq ) of Mg 2 FeH 6 and Mg 2 NiH 4 are ∼1.5 and 4 bar H 2 , respectively. 32 Thus, lower P sys (1.13 and 0.4 bar H 2 for S1 and S2, respectively) than P eq at these T eq encourages dehydrogenation of both samples. Aerwards rehydrogenation is carried out at isothermal condition (T set = 315°C) under 16 bar H 2 . By applying hydrogen pressure, T sample of both S1 and S2 enhance rapidly to T eq = 332 and 351°C, respectively, due to fast exothermic reaction (Fig. 3). Rehydrogenations of both samples complete within 11 min, assured by the reduction of T sample to the initial temperature. Under comparable P sys (16 bar H 2 ), different T eq values detected during hydrogenation of S1 and S2 suggest the alteration of reversible phases and reaction pathways. In the case of the 2 nd dehydrogenation, S1 and S2 reveal fast temperature reduction to comparable T eq , P sys , and reaction time of 308-311°C, 0.8-1.0 bar H 2 , and 10-11 min, respectively (Fig. 3). Aerwards, dehydrogenation kinetics, capacities, and reversibility upon 7 de/rehydrogenation cycles of S1 and S2 are investigated. During the 1 st dehydrogenation, hydrogen capacities of S1 and S2 are comparable of 3.2-3.3 wt% H 2 , but S1 shows faster dehydrogenation rate than S2 (Fig. 4). Considering the 2 nd dehydrogenation, kinetic properties of both samples are improved with respect to the 1 st cycle. Reversible capacity in the 2 nd cycle of S1 is maintained as 3.3 wt% H 2 , while that of S2 reduces to 2.8 wt% H 2 (Fig. 4). Upon the 3 rd -7 th cycles, kinetic properties of both samples are stable, but their storage capacities reduce to 2.7-2.8 and 2.4-2.6 wt% H 2 for S1 and S2, respectively.
Furthermore, phase compositions of S1 and S2 aer the 1 st de/rehydrogenation are investigated by PXRD technique. From  Fig. 5, the 1 st dehydrogenated S1 and S2 reveal comparable diffractions of Mg, Mg 2 Ni, Fe, MgO, and Fe-Ni alloy. Considering phase compositions of as-prepared and the 1 st dehydrogenated samples of S1 and S2, Mg and Fe are obtained from the  dehydrogenation of MgH 2 and Mg 2 FeH 6 (eqn (6) and (7)), while Mg 2 Ni is from the decomposition of Mg 2 NiH 4 (reverse reaction of eqn (5)).
Mg 2 FeH 6(s) / 2Mg (s) + Fe (s) + 3H 2(g) For the 1 st rehydrogenation, the formations of MgH 2 , Mg 2 NiH 4 , Mg 2 FeH 6 , and MgO are observed in S1. In the case of S2, the 1 st rehydrogenated sample reveals the diffractions of MgH 2 , Fe, Fe-Ni alloy, and MgO as well as Mg 2 Fe (1−x) Ni x H 6 , shown as a new diffraction peak locating between those of Mg 2 FeH 6 and Mg 2 NiH 4 . 23,25 The formations of MgH 2 , Mg 2 FeH 6 , and Mg 2 NiH 4 in S1 conrm rehydrogenation of Mg, MgH 2 + Fe, and Mg 2 Ni, respectively (reverse reactions of eqn (6) and (7) as well as eqn (5)). Besides, it was reported that Mg 2 NiH 4 was able to be synthesized by hydrogenating the mixture of coarsegrained Mg and Ni(Fe) nanoparticles and most of Ni(Fe) transformed to a-Fe when the reaction completed (eqn (8)). 33 Thus, the reduction of Fe-Ni alloy together with the increment of Fe aer the 1 st rehydrogenation of S1 (Fig. 5) can be explained by the reaction between Fe-Ni alloy and MgH 2 to form Mg 2 NiH 4 . In the case of the 1 st rehydrogenated S2, hydrogenations of Mg into MgH 2 (reverse reaction of eqn (6)) and Mg 2 Ni + Mg 2 FeH 6 into Mg 2 Fe (1−x) Ni x H 6 (eqn (9)) 23,25 are observed. Signicantly enhanced diffraction of Fe-Ni alloy and irreversibility of Mg 2 NiH 4 upon the 1 st hydrogenation of S2 suggest the increase of solid solution of Fe and Ni as well as no reaction between MgH 2 and Fe-Ni alloy (eqn (8)). Reaction pathways upon the 1 st de/rehydrogenation are summarized in Table 1. Due to the changes of reaction pathways and phases formed during the 1 st de/rehydrogenation of S1 and S2 (Fig. 5 and Table  1), temperature proles during the 1 st endothermic desorption and exothermic absorption of S1 and S2 are different (Fig. 3). Effective reproducibility of several hydrides in S1 (MgH 2 + Mg 2 FeH 6 + Mg 2 NiH 4 ) probably maintains reversible hydrogen capacities upon 2 cycles (∼3.3 wt% H 2 ) (Fig. 4A). Moreover, phase compositions of the 7 th rehydrogenated samples of S1 and S2 are characterized by PXRD technique to describe the reduction of hydrogen capacities upon cycling (Fig. 4). From Fig. 6, both rehydrogenated samples show comparable diffractions of MgH 2 , Fe, MgO, and unknown phase. Meanwhile, each sample shows different phases of Mg 2 FeH 6 + Mg 2 NiH 4 and Mg 2 Fe (1−x) Ni x H 6 for the 7 th rehydrogenated S1 and S2, respectively. Upon cycling, signicant amount of unreacted Fe with respect to the reversible hydrides is observed from both samples. The latter explains the decient hydrogen capacities of both samples upon the 3 rd -7 th cycles (Fig. 4).
To conrm the formation of Mg 2 Fe (1−x) Ni x H 6 in the 1 st and 7 th rehydrogenated S2, Fe 2p XPS experiments are carried out. From Fig. 7, Fe 2p XPS spectrum of as-prepared Mg 2 FeH 6 shows the characteristic peaks of Fe 0 (707. 5 Fig. 7(a) and (b)). This suggests the formation of another Fe-containing phase with lower oxidation state than 2+. Because the energy resolution of XPS measurements is 0.5 eV, the binding energy difference between Fe 2+ and Fe x+ (∼0.8 eV) is sufficient to imply that the energy shi is due to phase changes. Once partial substitution of Ni for Fe in Mg 2 FeH 6 to form Mg 2 Fe (1−x) Ni x H 6 occurs, the oxidation state of Fe reduces from  Fe 2+ to Fe x+ (0 < x < 2). Thus, the appearance of Fe x+ likely conrms the formation of Mg 2 Fe (1−x) Ni x H 6 in the 1 st and 7 th rehydrogenated S2.
Furthermore, it should be mentioned that phase compositions in the 1 st dehydrogenated samples of S1 and S2 are comparable (i.e., Mg, Mg 2 Ni, Fe-Ni alloy, and Fe) (Fig. 5). However, the reaction pathways during the 1 st rehydrogenation of these samples are different, affecting reversible hydrogen capacities (Table 1 and Fig. 4). This might relate to contacts and distribution of the reactive phases in the bulk samples. Therefore, microstructural analyses of the 1 st dehydrogenated S1 and S2 are investigated by TEM, electron diffraction, and EDS mapping. TEM image of the 1 st dehydrogenated S1 shows that at least two different phases are well distributed in the nanometer scale (Fig. 8A(a)). The corresponding SAED pattern conrms the presence of Mg, Mg 2 Ni, and Fe-Ni (Fig. 8A(b)), in accordance with PXRD result (Fig. 5). EDS maps reveal excellent distribution of Mg, Fe, and Ni in the sample bulk (Fig. 8A(c) and (e)). These results suggest good contacts among Mg, Fe, Mg 2 Ni, and Fe-Ni in the 1 st dehydrogenated S1. This likely promotes the formation of Mg 2 FeH 6 and Mg 2 NiH 4 upon rehydrogenation ( Fig. 5 and Table 1). In the case of the 1 st dehydrogenated S2, TEM micrograph shows signicant particle agglomeration ( Fig. 8B(a)) with comparable phase compositions to S1 (SAED result in Fig. 8B(b)). From EDS maps, Mg and Ni occupying comparable location show well-distributed nanoparticles with partially dense agglomeration (Fig. 8B(c) and (e)), while Fe shows good distribution of sintered particles (Fig. 8B(d)). These distributions either of nanoparticles or sintered particles found in Mg, Ni, and Fe maps lead to the homogeneous reversibility of MgH 2 , Mg 2 FeH 6 , and Fe-Ni alloy all over the sample bulk. The positions with Mg and Ni agglomeration, probably containing high density of Mg 2 Ni benet for hydrogenation of Mg 2 FeH 6 + Mg 2 Ni to form Mg 2 Fe (1−x) Ni x H 6 (eqn (9)). Thus, using different Ni sources (metallic Ni or Mg 2 NiH 4 ) as staring material affects the contacts among active phases. S1 using metallic Ni shows better distribution of metal nanoparticles than S2, which Ni is from Mg 2 NiH 4 . The 1 st dehydrogenated S1 with good metal distribution reproduces individual hydrides (Mg 2 FeH 6 and Mg 2 NiH 4 ) upon rehydrogenation. For the 1 st dehydrogenated S2, agglomeration of Mg 2 Ni (from direct decomposition of Mg 2 NiH 4 ), which is in good contacts with Mg and Fe favors the formation of Mg 2 Fe (1−x) Ni x H 6 . Therefore, the distribution and contacts among metal nanoparticles results in different reaction pathways upon de/rehydrogenation and reversible hydrogen capacities.

Conclusions
The effects of Ni precursors (metallic Ni or Mg 2 NiH 4 ) on the formation of Mg-Fe-Ni hydrides were studied and the de/ rehydrogenation kinetics and reversibility of the obtained samples were investigated. The mixtures of MgH 2 + Mg 2 FeH 6 + Mg 2 NiH 4 and Mg 2 FeH 6 + Mg 2 NiH 4 were obtained from the asprepared samples with metallic Ni and Mg 2 NiH 4 , respectively. Although both samples released comparable hydrogen during the 1 st cycle (3.2-3.3 wt% H 2 ), the reduction of dehydrogenation temperature (DT = 12°C) and faster kinetics were obtained from the as-prepared sample with metallic Ni. Aer the 1 st dehydrogenation, similar phase compositions of Mg, Mg 2 Ni, Fe, and Fe-Ni alloy were found in both samples. Nevertheless, different Ni precursors altered phase compositions and reaction pathways during rehydrogenation. The reversible phases of the sample with metallic Ni were MgH 2 , Mg 2 FeH 6 , and Mg 2 NiH 4 , while those of the sample with Mg 2 NiH 4 were MgH 2 and Mg 2 -Fe (1−x) Ni x H 6 . These recovered phases affected reversible capacities. For example, hydrogen capacities during the 2 nd -7 th cycles of the sample with metallic Ni were 2.7-3.2 wt% H 2 , while those of the sample with Mg 2 NiH 4 reduced to 2.4-2.8 wt% H 2 . Decient reversible capacities of both samples, especially aer the 3 rd cycles could be described by signicant amount of unreacted Fe. Considering microstructural analyses, the sample with metallic Ni contained well-distributed nanoparticles of all metals, beneting for individual reversibility of MgH 2 , Mg 2 FeH 6 and Mg 2 NiH 4 . For the sample with Mg 2 NiH 4 , partial agglomeration of Mg and Ni at comparable location, likely belonging to Mg 2 Ni favoured the formation of Mg 2 Fe (1−x) Ni x H 6 . Due to the recovery of multiple hydride phases, hydrogen capacities upon cycling of the sample with metallic Ni was signicant.

Conflicts of interest
There are no conicts to declare.